Ultima Online is arguably the father of the whole MMO scene. There were other mmos at the time (Neverwinter Nights online was on AOL starting in the 90s) but none which showed us the potential of this as a gaming platform until Richard Garriot gave us Ultima Online.
I remember the launch; I spent most of the night trying to log in, server problems, lag problems, login issues. You name the problem; they had it. But those few minutes of play time just made the down-time worth it.
Anywhere PVP, portal runes, dyes, player housing, etc. Great stuff. Back when dying meant quite likely losing your equipped gear.
I'm just hoping Ultima Forever polishes up what we all loved about the title; without changing the game too much. Give us a world worth exploring and exploiting! Give survival meaning!